The present invention relates a phase shift device with at least one tunable component. This phase shift is frequency independent (phase shifter) or frequency dependent (variable delay line).
The shortage in the available frequency spectrum for radio communications and the requirement for more functionality in smaller volume increase the demand for reconfigurable components. Hereinafter radio frequency (RF) is meant a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves and of the alternating currents which carry and transmit radio signals. Depending on the device requirements there are different possible solutions for phase shift devices like e.g. semiconductors, MEMS or tunable dielectrics to design agile RF components.
Phase shift devices are one of the key elements for reconfigurable electronic beam steering antennas.
From the state of the art the following documents are cited as examples of the above mentioned phase shift devices:    1. U.S. Pat. No. 8,305,259 B2    2. U.S. Pat. No. 8,022,861 B2    3. U.S. Pat. No. 8,013,688B2    4. PCT patent application WO 2012/123072 A1    5. US patent application US 2009/0302976 A1    6. F. Goelden, A. Gaebler, M. Goebel, A. Manabe, S. Mueller, and R. Jakoby, “Tunable liquid crystal phase shifter for microwave frequencies,” Electronics Letters, vol. 45, no. 13, pp. 686-687, 2009.    7. O. H. Karabey, F. Goelden, A. Gaebler, S. Strunck, and R. Jakoby, “Tunable 5 loaded line phase shifters for microwave applications,” in Proc. IEEE MTT-S Int. Microwave Symp. Digest (MTT), 2011, pp. 1-4.    8. U.S. Pat. No. 5,936,484 A    9. Japanese patent application JP 2003/008310 A    10. Onur Hamza Karabey et al., “Continuously Polarization Agile Antenna by Using Liquid Crystal-Based Tunable Variable Delay Lines”, IEEE vol. 61, no. 1, 1 Jan. 2013, pages 70-76. ISSN: 0018-926X    11. US patent application US 2009/073332 A1
Microwave components such as phase shift devices can be formed from microstrip lines. A microstrip line is a type of electrical planar transmission line which can be fabricated using printed circuit board technology. It consists of or comprises a conducting strip shaped electrode that is separated from a planar ground electrode by a dielectric layer known as the substrate.
As in [1, 3], the substrate can be formed by liquid crystal polymers (LCPs). However, the LCP material is not tunable with respect to the relative permittivity of this material. Therefore, due to the lack of tunable characteristic features that are relevant for the configuration of a tunable RF device, making use of LCPs is not convenient for designing of a tunable device such as a varactor.
Liquid crystals (LCs) exhibits features and properties of conventional liquids as well as of solid crystals. For instance, an LC may flow like a liquid, but its molecules may be orientated in a crystal-like way. In contrast to liquid crystal polymers (LCPs), the relative permittivity of liquid crystals (LCs) may be affected by e.g. a voltage applied to the LC material.
In prior art [6], coplanar waveguides (CPW) have been loaded with shunt LC varactors. The tuning speed of such an LC devices is affected by its topology. One of the known drawbacks of such devices is the high metallic loss of the CPW. Additionally, due to the CPW, the varactors were implemented by patches as floating electrodes bridging the center conductor and the ground plane. This results in reduced tuning efficiency and high insertion loss of such an artificial transmission line.
Similarly in [7], the tuning efficiency of the LC varactors is improved by using a parallel plate capacitor topology. However, there a slotline was loaded with these LC varactors in shunts. Indeed, loading a microstrip line results in higher performance because microstrip lines feature system-inherently low losses.
Therefore it is desirable to reduce disadvantages of phase shift devices according to prior art and to provide for a favorable phase shift device with low response time as well as with a high performance, that comprises both features simultaneously in a compact and planar configuration.
The invention relates, according to an aspect thereof, to a phase shift device that is formed by a signal electrode an a ground electrode which are separated by a dielectric substance, and further comprising a liquid crystal material, characterized in that the signal electrode of the planar transmission line is divided into several pieces and comprises overlapping areas of adjacent pieces that are filled with a tunable liquid crystal material, thereby forming a dielectric tunable component (varactor) with a metal-insulator-metal type capacitor.
A transmission line (microstrip line) is formed by two electrodes: A signal electrode and a ground electrode. The material of the electrodes is preferably low resistive RF electrode material such as Ag, Cu or Au. Other materials or alloys with similar conductive properties are possible. The signal electrode is divided into several pieces along the length, i.e. along the propagation direction. These pieces are e.g. implemented as top side pieces on the top side of bottom glass and as bottom side pieces on the bottom side of top glass. The top glass and the bottom glass are stacked in such a way that a in plan view, i.e. when viewed perpendicular to the propagation of the signal a continuous signal electrode is formed. The terms top glass and bottom glass do not indicate that other suitable materials are waived. Additionally, there are some sections where the top side pieces and the bottom side pieces of the signal electrode are overlapping. At least, these overlapping areas between the two glasses and the top side piece and the respective bottom side piece of the signal electrode are filled with a tunable liquid crystal material. Therefore, each overlapping area forms a metal-insulator-metal type capacitor. In this case, since the insulator is the tunable liquid crystal material, this area forms a dielectric tunable component (varactor). This tunable component enables to construct this phase shift device in a very compact way.
Due to LC technology, the varactors feature low losses compared to other technologies such as semiconductors for frequencies higher than 5 GHz preferably frequencies higher than 10 GHz. Additionally, since microstrip line is used, the phase shifter losses would be much less than compared to [6, 7].
According to an embodiment of an aspect of the invention the several pieces of the signal electrode are arranged at two or more different distance levels with respect to the ground electrode. Arrangement of the several pieces of the signal electrode at two different levels allows for easy and cost-saving fabrication of such a device, as the two different levels can be on two surfaces of substrate layers. Three or more different distance levels allow for complex configurations and e.g. different layers of LC material that is located between two adjacent distance levels.
The tunable liquid crystal material can be arranged as a single and continuous layer between several pieces of the signal electrode that are arranged at two different distance levels. The boundaries of this continuous, i.e. unbroken layer can be adapted and limited to the shape and extension of the signal electrode that is defined as a composed covering of the several pieces. This continuous layer can completely cover the usually larger ground electrode. For many applications the continuous layer can be arranged between two adjacent layers of either electrodes or substrate layers and completely fill a cavity between those dielectric substrate layers. This allows for fast and cheap fabrication of such an arrangement e.g. by use of well-established liquid crystal display technology.
However, in order to save tunable liquid crystal material or to allow for separate control of confined spatial regions of the tunable liquid crystal material it is possible to arrange the tunable liquid crystal material as several confined layer areas between the overlapping areas of adjacent pieces of the signal electrode at two different distance levels.
For most applications arranging the pieces of the signal electrode parallel to the propagation direction, e.g. linearly along the propagation direction of a radio frequency signal is advantageous, since this prevents any discontinues, resulting in less losses. If required or feasible, the pieces of the signal electrode are arranged in a straight line.
However, for some applications that require many varactors along the transmission line, the transmission line may be meandered, e.g. in an N-shape or in spiral shape. This allows for a transmission line length that is much longer than the physical dimension of the phase shift device.
The phase shift along the transmission line is exclusively or at least predominantly due to the tunable varactors that are of the metal-insulator-metal capacitor type and that are arranged along the signal electrode. The configuration, shape and arrangement of the pieces of the signal electrode should not result in resonant structures that significantly affects the time delay for the signal propagation along the transmission line.
A phase shift device according to an embodiment of an aspect of the invention can be characterized in that the signal electrode is divided into several pieces along the length of the transmission line, whereby said several pieces are alternatingly implemented as top side pieces on the top side and as bottom side pieces on the bottom side of a non-tunable dielectric substrate and whereby at some sections there are overlapping areas between an top side piece and an adjacent bottom side piece of the signal electrode, and whereby these overlapping areas are filled with a tunable liquid crystal material and these overlapping areas form a dielectric tunable component (varactor) with a metal-insulator-metal type capacitor.
In a exemplary embodiment the non-tunable dielectric substrate that supports the several pieces of the signal electrode is chosen to be a 700 μm thick borofloat glass from Schott AG with εr,glass=4.6 and its loss tangent tan δ=0.0037 at 25° C. and at 1 MHz. A LC mixture is used, whose relative dielectric constant is continuously tunable between 2.4 and 3.2 by application of a tuning voltage and surface alignment method. The maximum dielectric loss tangent tan δ of this material is less than 0.006 for all tuning states. The LC layer (from metal to metal) is specified to 3 μm to obtain a fast response time that is less than 25 ms. The device provides for a 367° differential phase shift at 20 GHz with 6.1 dB insertion loss at maximum.
An important parameter for quantifying the RF performance of these devices is a frequency-dependent figure of merit (FoM). This is defined by the ratio of the maximum differential phase shift and the highest insertion loss over all tuning states.
Hence, the FoM of the exemplary embodiment is 60°/dB at 20 GHz.
In a further embodiment the planar transmission line comprises at least two serially connected dielectric tunable components that are connected by a non-overlapping section of the signal electrode. The signal transmission along the planar transmission line is dominantly and basically solely affected be the number and configuration of the varactors, i.e. the tunable dielectric components that are arranged along the signal electrode. The phase shift is easily controlled and modified by application of a tuning bias voltage to the tunable liquid crystal material that forms the tunable dielectric material between the overlapping areas of adjacent pieces of the signal electrode, i.e. the parallel plate like tunable capacitor that acts as the varactor.
The tuning is performed by control electrodes. These electrodes operate as control element. These electrodes transmit the different bias voltages to drive the varactors through bias lines. The bias lines are preferably made of a low conductive material in order not to affect the RF circuitry. For this purpose, low conductive electrodes can be used since they become transparent for the RF signal. Typical materials for the bias lines are preferably ITO (indium tin oxide), NiCr (nickel chromium) or some other alloys having a conductivity of less than 10e5 S/m.
In further embodiments the phase shift device is combined with a radiating element to transmit the RF signal. Often radiating element are also referred to as patch antennas. The radiating elements and the feed lines are usually photoetched on the dielectric substrate. The radiating elements, i.e. the patch antennas are configured as square, rectangular, thin strip (dipole), circular, elliptical, triangular, or any other formation.
In further embodiments the radiating element is an arbitrarily shaped microstrip patch antenna or a microstrip slot antenna.
Liquid crystals (LC) are suitable to realize tunable RF devices. LC can be employed as a tunable dielectric, specifically optimized LC mixtures offer high performance at microwave frequencies with a loss tangent below 0.006. The relative tunability, defined as the ratio of the tuning range of the minimum permittivity to the maximum permittivity, is preferably between 5% and 30% or between 10% and 25% or between 15% and 30% or between 5% and 14%.
LC based planar phase shift devices are usually customized depending on desired antenna performance. For this purpose the phase shift device is configured and adapted in order to reduce insertion loss, to increase beam steering speed and to allow for wide range scans. According to this invention, a LC mixture optimized for RF applications is used. A possibility to realize tunable RF components with LC is presented in FIG. 3. It shows the cross section of an inverted microstrip line using LC as a tunable substrate for different bias voltages. The setup consists of or comprises two stacked substrates, the top one carrying the microstrip line and the lower one the ground plane. In between the two substrates, a thin LC layer is encapsulated.
The phase shift device according to an aspect of the invention can be combined with a radiating element e.g. in order to provide for phased array antenna.
In a first embodiment of such a combination the planar transmission line and the antenna are coupled by using an aperture coupling method. In a second embodiment the planar transmission line and the antenna are coupled by using a proximity coupling method. In a third embodiment the planar transmission line and the antenna are coupled directly, e.g. using a inset-fed technique or through an vertical interconnect.